Image display modules of electronic devices such as portable phone terminals, computer displays, and the like, as well as optical members such as touch panels, often have glass or a plastic film laminated thereupon as a surface-protecting layer. Such surface-protecting layers are affixed to the image display module or touch panel via the application of a frame-shaped tape or an adhesive to an outer blank area of the image display part or outside of the effective operating area of the touch panel. As a result, a space is formed between the image display part or effective operating area of the touch panel and the surface-protecting layer.
In recent years, a method of improving the transparency and increasing the clarity of an image has become widely used, in which the space between the surface-protecting layer and the image display module or touch panel is replaced by a transparent material such that the difference in refractive index between the display surface of the surface-protecting layer, touch panel, and image display module is lower than that of air (in other words, a transparent material with a refractive index close to that of glass or plastic). Examples of such transparent materials include pressure-sensitive adhesives, adhesives, silicone gels, and the like. If an adhesive is used, it can be difficult to apply the adhesive only to a specific area, and expensive equipment may be needed for application. In addition, peeling or adherend warping can result from stress generated from contraction, if an adhesive requiring curing is used. Silicone gels present problems of long-term reliability due to low adhesiveness. In contrast to this, a pressure-sensitive adhesive (for example, a pressure-sensitive adhesive (PSA) sheet) can be processed beforehand to a predetermined shape and then applied, has sufficient adhesive strength, and can be reapplied, making such a material effective for applying a surface-protecting layer to an image display module or touch panel.
The surface of an adherend such as an image display module, optical member, or surface-protecting layer may not be flat. The surface of the surface-protecting layer, especially a surface coming in contact with an adhesive sheet, may be printed upon for decorative or light-blocking purposes. In a number of examples, the printed portion may form a three-dimensional surface topography, such as a 10 μm-high raised section, on a surface of the surface-protecting. When an adhesive sheet is used to apply a surface-protecting layer on an image display module or touch panel, there is the problem, for example, that insufficient conformity to the three-dimensional surface topography by the adhesive sheet can lead to spaces being formed upon or in the vicinity of the three-dimensional surface topography. Moreover, warping from the stress caused by deformation of the pressure-sensitive adhesive can cause color patches in adherends that are sensitive to warping, such as LCDs. In order to avoid these problems, it is normally necessary to impart the adhesive sheet with a thickness approximately ten times the height of the three-dimensional surface topography. If a pressure-sensitive adhesive with poor stress alleviation properties is used, the quality demanded when applied may not be met even if the thickness of the material is ten times or more the height of the three-dimensional surface topography.
In addition, there is the problem that, when a thick (for example, 50 μm or greater) adhesive sheet is produced, air bubbles are prone to form in the adhesive sheet itself due to the thickness thereof. Air bubbles are particularly prone to form when the adhesive sheet is produced via steps of applying a fluid, constituted by a pressure-sensitive adhesive component dissolved in a solvent, to the substrate and drying the solvent.